If you can find replacement inserts use them, however in my 50 year experience I have never found them. If you buy a set of complete blades, and if they also sell replacement inserts for them I suggest you buy a couple of sets of inserts at the same time because when you need them they will no longer be available.

I have had good luck in extending the life of wipers by scrubbing the glass with BAR KEEPERS FRIEND every few months